In recent years, with the trend toward electronic devices having a smaller size and higher performance, there has been a need for a higher density of printed circuit boards. As a substrate for a printed circuit board that satisfies such a need for a higher density, there has been a demand for a substrate for a printed circuit board in which conductive layers have smaller thicknesses.
In order to meet the demand, a substrate for a printed circuit board has been proposed in which a thin copper layer is formed on a heat-resistant insulating base film without using an adhesive layer therebetween (refer to Japanese Patent No. 3570802). In this conventional substrate for a printed circuit board, thin copper layers (first conductive layers) having a thickness of 0.25 to 0.30 μm are formed on both surfaces of a heat-resistant insulating base film by sputtering and, on the layers, thick copper layers (second conductive layers) are formed by electroplating.